I might be misunderstanding something, but doesn't Railgun require two technologies - a Railgun tech and a Railgun Launch Velocity tech? If you don't have any Railgun tech yet you'd be missing the latter.
It's like how you need three different technologies to make your first jump drive.
I did wonder this but ive not got the techs required and i still cannot seem to make the more advanced tech components that i acquired from the ruins.
Wait you said
10cm Advanced Railgun? Is this some sort of ruins only tech that isn't properly implemented yet?
I might be confused but if you got
normal railgun tech it should just be called
"10cm Railgun".
The advanced variant sounds like it might be something else entirely. Can you discern any sort stats from what little the game has told you about it or do you just know the name?
Also for completeness double check whether this is a tech or some component you found - this could just be
10cm Railgun that is just called a
10cm advanced railgun.
Edit: Also make sure that it is fully researched - go to the research tab on the population that you found the tech and check whether or not the tech has been
partially researched as opposed to fully.